The subject invention is a shipboard radar/antenna interface coupling system. Such systems, in which radio frequency (r.f.) signals must pass to the antenna from a transmitter located below decks, are well known in the art. In this particular type of radar system, the antenna rotates through 360 degrees of azimuth coverage. Therefore, a rotary coupling system is needed.
Shipboard radars require stabilized systems to maintain elevation coverage and also for beam steering to find height and target location. Innumerable techniques have been used, utilizing either large mechanical devices or active electronics on the antenna. These approaches often have reliability and maintenance problems. Radar systems having a rotary array with control of individual rows to form elevation beams, but having only straight-forward azimuth rotation, represent a sound, economical design. To simplify maintenance it is desirable that outputs from each row be brought off the rotating platform so that the electronics can be accessible and protected below decks. The present emphasis on solid state transmitters that combine the outputs of dozens of modules in a given space allows an alternative to mechanical devices and active antenna electronics: multiple rotary joints (see FIG. 1). Elevation steering is electronic, and takes place below decks. Each array row 2 is fed by at least one module with controllable phase. The phase shift can be accomplished at low power. By thus eliminating the electrical loss due to both the phase shifters and their combiners, the transmission line loss in the run up the mast becomes less critical. To be successful, this method requires multiple, high power, low loss, and phase stable rotary joint paths.
All 360 degree rotary joints must provide a circularly symmetric junction between the stationary and rotary paths, so that rotation can take place without signal variations caused by the coupling the term (WOW) as used in the industry as a variation of an electrical property of a rotary device as a function of rotation of the device through 360 degrees. Further, with N multiple paths, N-1 transmission lines must pass inside the largest coupler.
For up to three high power channels, concentric coaxial rotary joints of progressively smaller diameters are commonly used, with progressively less power handling from outer to innermost joint. Beyond three paths, stacking must be done vertically instead, using thin "pancake" joints 3. These have oversized, inner diameter holes 4 sufficiently large to allow the passage of all transmission lines 5 from the joints 3 to the antenna above them, as illustrated in FIG. 2a and 2b. This usually makes the circumference large in number of wavelengths, requiring multiple drive points around the joints 3 to obtain a uniform field at the circular junction. In order to transfer energy across this large diameter junction without sensitivity to rotation, each side must be uniformly driven. If the excitation points are spaced more than one wavelength apart on the circumference, at least two modes will propagate unattenuated at different velocities, and drastic variations with azimuth will occur. Even if driving points are closer than a wavelength, the attenuation to non-propagating modes is limited for short lengths of coax, so carefully designed input and output networks are required to meet a specified WOW level. Clearly the larger the diameter the more complex the driving network. For high power level and wide bandwidth, 15/8" diameter coax might typically be required for input and output lines such as transmission lines 5. As shown in FIG. 5a, if densely packed into the inner diameter hole 4, at least a 10" diameter hole 4 would be required for the specific case considered herein in detail, using 21 high power lines and 6 low power lines, along with different styles of connections depending on where the coax was placed. If all were at a single radius, the diameter of hole 4 would reach 13", which would create serious design problems. The number of drive point taps required is directly proportional to the diameter, and the divider size is proportional to both, making the diameter of hole 4 the primary determinant of rotary joint unit size and weight
Usually the entire outside of the rotary coupler is the stationary component, while the inside section rotates with the antenna. The input line divides into enough drive point taps to drive a low impedance coaxial line. The output has multiple taps as well to collect the signals, which are recombined at the antenna port. The rotor-to-stator junction itself must be a non-contacting type to allow rotation and to have significant life at high power. Normally a multiple section choke joint is employed to realize the junction, with the section impedances chosen to provide low VSWR across the band.
Such rotary couplers are not new to the art, but are usually associated with multiple low power applications, for the following reasons. If the many channels are to fit within the above decks antenna platform, they must be of minimum height and less than a specific diameter. Referring to FIGS. 3a and 3b, which show two planar cross-sections of the current state of the art in pancake joints, minimizing the size of joints is seen to be quite difficult. The joint has coaxial inputs and outputs, and uses many layers of microwave circuitry. In FIG. 3a, starting at the bottom, there is an inner conductor choke 6, an outer divider 8, a choke on the divider balun 10, an outer conductor choke 12, a combiner balun 14, a combiner 16, and a return of the combiner to the center 18. Each joint 3 is tied to the low impedance coax and each layer will function more effectively as its thickness increases, both in power handling and in bandwidth. With 1" striplines and 0.5" chokes provided to achieve desired power handling and bandwidth characteristics, 5" minimum thickness is required for each channel. This makes the system as it exists prohibitively large for high power applications. In high power applications, excessive heat may also be a problem in these prior art systems. Excessive heat must be avoided especially, as liquid cooling can be difficult on board ship; any cooling system used must be kept simple and reliable.
Since each of these prior art channels contains chokes, baluns, and transformers, bandwidth is often limited and losses are moderately high. Large conductors can be used, as they have a higher peak power capacity. However, the large dimensions strain the size and weight limitations and put a premium on optimal location of each part and on mechanical design. Also, larger conductors can be significant fractions of a wavelength wide, complicating the already difficult task of microwave design over the wide bandwidth.
Also, in most conventional designs of pancake modules, each channel is entirely self contained, with its own bearing or bearings. Assembly of several channels into a package involves stacking the required number of modules one above another, with some method of tying the stators and rotors of each module to its neighbors. This process necessarily gives rise to concentricity and alignment problems. Also, because the individual module bearings are relatively small, and undergo continuous use, bearing loads can become quite high, with resultant short bearing life.
In addition, when individual module bearings are used, the module design is often complicated by the need to shield the bearings from r.f. energy, particularly in a high power module. Improperly shielded bearings may arc in the presence of r.f., seriously reducing their lifetimes, or resulting in catastrophic failure. To protect them requires extensive choke and load designs modification.
What is needed is a design that will accommodate a high average power r.f. signal with a large bandwidth, that is of compact size and weight, and that suffers low losses in order to keep temperatures reasonable and has facilities for cooling to permit high power level use.